1. Field of the Invention
The present invention relates to an aromatic polyimide film and a process for the preparation of the same. More particularly, the invention relates to an aromatic polyimide film which is resistant to sagging and has excellent surface conditions (highly plane surface) as well as producing less dimensional change after heat treatment, and a process for the preparation of the same.
2. Description of Prior Art
Since a previously known aromatic polyimide film such as prepared using a pyromellitic acid shows poor dimensional stability when it is heated, extremely heavy curling is brought about when the film is combined with other material such as a copper foil at high temperatures. For this reason, there has been proposed a method of performing a stretching treatment (i.e., orientation) in the film-forming process to reduce the curling (Japanese Patent Publication No. 44(1969)-20878).
Recently, an aromatic polyimide film originating from a polymer (i.e., polyamic acid) produced by polymerization reaction of a biphenyltetracarboxylic acid component such as 3,3',4,4'-biphenyltetracarboxylic dianhydride and a phenylenediamine component such as p-phenylenediamine has been developed and confirmed to have a low linear expansion coefficient. Accordingly, a polyimide film of this type is expected to show less curling. However, the aromatic polyimide film is insufficient in the dimensional stability under application of heat, though heavy curling can be avoided, so that various problems still remain. For example, when the aromatic polyimide film is combined with other thin film such as a ceramic film or a conductive metal foil to prepare a composite material (i.e., laminate material, the polyimide film easily deforms in the heating process such as an etching process and a soldering process.
In view of the above-mentioned problem, there has been proposed processes for the preparation of an aromatic polyimide film in which dimensional change is hardly brought about even after the heat treatment, as described in U.S. Pat. No. 4,725,484.
The processes described in the above U.S. Patent are developed for solving the aforementioned drawback, that is, solving the problem of dimensional change. One process described in the U.S. Patent comprises the steps of drying a thin film of a polyamic acid solution coated over a support to to obtain a solid film (namely, a film containing a small amount of a solvent), then heating the film to further dryness under the condition that both end portions of the film in the longitudinal direction are fixed to obtain an aromatic polyimide film, and finally again heating the resulting polyimide film at a high temperature under weak tension to mitigate the stress produced in the imidation reaction.
In another process described in the U.S. Patent, the drying process of a thin film of a polyamic acid solution formed on a support is performed slowly in two stages, that is, a first stage of drying the thin film with the support and a second stage of drying the thin film after separation from the support under weak tension, to obtain a solid film which is reduced in occurrence of stress. Thereafter, the resulting film is finally heated to dryness to perform imidation reaction under the condition that at least a pair of end portions of the film are fixed.
By conducting the above-mentioned heat treatments described in the U.S. Patent, the aromatic polyimide resin film is prominently enhanced in the dimensional stability in the heating process. However, the present inventors have found that the side edge portions of the film in the transverse direction are put under restraints but the central portion of the film in the same direction is placed under no specific restraint in the final heating procedure, namely, the heating procedure of mitigating the stress to fix the film form in the former process or the heating procedure of restraining the stress to fix the film form in the latter process, and hence the film form is fixed under such condition that a difference is given between the side edge portion of the film and the central portion thereof with respect to the shrinkage rate in the longitudinal direction. It has been confirmed that the shrinkage of the side edge portion of the film in the longitudinal direction is larger than that of the central portion in the same direction, whereby sagging is easily brought about in the central portion of the film. The occurrence of such sagging causes a problem of non-uniform adhesion to other material in the case of using the aromatic polyimide film for producing the aforementioned composite material (i.e., laminate material). Accordingly, it is eagerly required to prevent occurrence of sagging in the aromatic polyimide film.